LED Intensity Decay Particle Tracking Velocimetry (PTV)
Instrumentation
LED Intensity Decay Particle Tracking Velocimetry (PTV) (LAR-TOPS-394)
A cost-effective, high-resolution alternative to laser-based PTV systems
Overview
Particle tracking velocimetry (PTV) is a common technique used for measuring fluid flow by tracking the motion of small, micron-sized particles seeded in a fluid medium. Traditionally, PTV and its related technique, particle image velocimetry (PIV), rely on high-powered pulsed lasers that generate thin, intense light sheets to illuminate the particles. A high-speed camera captures sequential images, and advanced processing algorithms determine velocity vectors based on the displacement of the particles over time. While effective, these laser-based systems require expensive components, precise optical setups, and complex synchronization between the laser pulses and the camera, making them costly and challenging to implement.
Innovators at NASA’s Langley Research Center (LaRC) have developed LED Intensity Decay Particle Tracking Velocimetry (LED-ID PTV), an alternative to conventional PTV that uses light-emitting diodes (LEDs) for flow illumination. LED-ID PTV leverages the natural capacitive intensity decay of LEDs to encode velocity and directional information passively. This approach eliminates the need for expensive cameras capable of double-pulsing exposures, lasers, and intricate timing mechanisms while still delivering high-resolution flow field data. The system is cost-effective, eye-safe, and adaptable to various experimental conditions, making it ideal for aerospace, industrial, and research applications.
The Technology
NASA’s LED-ID PTV system illuminates a seeded flow with an LED rather than a laser. Instead of using double-pulsed laser flashes to capture two separate images of particle positions, the system relies on the inherent intensity decay of an LED pulse to encode velocity information directly into a single long-exposure image. The LED’s light intensity decreases over time due to capacitor discharge characteristics of the driving circuit. This controlled decay serves as a built-in intensity marker, allowing for precise determination of particle velocity and directionality without requiring an actively modulated light source.
In a single-color configuration, a monochrome camera captures a long- exposure image of particle streaks as they move through the illuminated region. Because the light intensity is continuously decreasing, the recorded streaks naturally encode velocity information based on their brightness gradient. Faster-moving particles create longer streaks, while slower particles form shorter ones. The intensity variation across the streak provides additional data about directionality, enabling flow field analysis with a minimal hardware setup. For more complex flow analysis, a two-color configuration can be employed to track three- dimensional motion. In this setup, two LEDs of different colors are positioned adjacent to each other to create overlapping light sheets. A color camera, or two monochrome cameras with a dichroic mirror, captures the streaks of particles as they move between these sheets.
The color transition within a particle’s streak indicates its movement between the planes of illumination, allowing users to resolve out-of- plane velocity components. Image processing techniques (e.g., advanced algorithms, high-pass filtering methods, sub-interval streak segmentation) further enhance the system's accuracy.
NASA’s LED-ID PTV system has been prototyped and demonstrated with excellent results, and is available for patent licensing to industry.
Benefits
- Cost-Effective: NASA’s LED-ID PTV system eliminates the need for expensive lasers and cameras capable of double-pulsing exposure. Equipment costs for a NASA prototype system were less than $1,000.
- Improved Safety: Using LEDs in lieu of high powered, pulsed lasers reduces eye damage and electrocution risks.
- 3D Flow Tracking Capabilities: By using two different color LEDs to illuminate adjacent or overlapping regions of the flow, NASA’s system can track not only in-plane (2D) velocity components, but also the out-of- plane velocity component - using only a single camera.
- Provides Particle Direction Information: Conventional particle streak imaging shows the particle trace over a specified time duration, but the direction of the particle is unknown. Using NASA's LED-ID PTV, the direction of the particle motion is known (in the direction of decreasing intensity).
- Simplified Imaging Setup: LED-ID PTV requires only an LED with its driving circuit and a standard camera.
- Adaptable to Different Flow Speeds: Circuit capacitance can be changed to tailor the decay rate for different particle velocity ranges.
- Flexible experimental configurations: LED-ID PTV works with single- or dual-camera setups, depending on imaging needs.
Applications
- Aerospace: Measuring velocity fields in aerodynamic experiments such as wind tunnel testing.
- Biomedical Flow Studies: Visualizing fluid flow in microfluidic and biological applications.
- Industrial Fluid Dynamics: Assessing airflow in HVAC, combustion systems, and chemical reactors.
- Environmental Studies: Analyzing air or water flow in atmospheric or hydrological research.
- Automotive Aerodynamics: Investigating airflow over vehicles for fuel efficiency improvements.
- Educational and Research Labs: Providing a cost-effective alternative for flow visualization experiments.
Technology Details
Instrumentation
LAR-TOPS-394
LAR-20413-1
Patent Pending
“LED Intensity Decay Particle Tracking Velocimetry,” Joshua M. Weisberger and Brett F. Bathel, NASA Langley Research Center, 07/2024, https://ntrs.nasa.gov/citations/20240007419
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